Addiction is a serious public health problem in the United States with opiates ranking among the top drugs of dependence. Stress has been implicated as a risk factor in vulnerability to the initiation and maintenance of opiate abuse and is thought to play an important role in relapse in subjects with a history of abuse. Conversely, chronic opiate use and withdrawal are stressors and can potentially predispose individuals to stress-related psychiatric disorders. Because the interaction of opiates with stress response systems has potentially widespread clinical consequences, it is important to delineate how specific substrates of the stress response and endogenous opioid systems interact and the specific points at which stress circuits and endogenous opioid systems intersect. The locus coeruleus (LC)-norepinephrine (NE) system is reciprocally regulated by endogenous opioids and the stress-related neuropeptide, corticotropin-releasing factor (CRF). Our prior studies have shown that chronic morphine exposure sensitizes the LC-NE system to CRF and stress, providing a potential mechanism that could link opiate use and vulnerability to stress-related psychiatric disorders. Withdrawal from opiates engages CRF and other stress-related systems including noradrenergic pathways, which produce heightened anxiety-like states and dysphoria that can increase susceptibility to relapse. Our research has unveiled the complex circuitry by which CRF and endogenous opioids co-regulate activity of the LC-NE system. Our findings indicate that CRF and enkephalin (ENK, acting at mu-OR) exert a postsynaptic opposing regulation. Our most recent studies revealed a novel presynaptic regulation of afferent inputs via kappa-OR modulation of excitatory (glutamate) and CRF afferents to the LC. The circuitry that links these peptides to the LC-NE system and the conditions that engage this circuitry were identified and highlighted the central nucleus of the amygdala as a key structure in its afferent regulation. Additionally, we demonstrated that chronic morphine disturbs this regulation. Given how LC activity is finely tuned by the integration of CRF and endogenous opioid inputs, upsetting the CRF:opioid balance in the LC could influence the stress-sensitivity of this system or its sensitivity to opiates. In the competing renewal application, mechanistic studies are proposed to elucidate how chronic morphine or chronic stress dysregulate the LC-NE system and its consequent behavioral implications. AIM 1 will identify mechanisms by which LC neurons become dysregulated by chronic morphine at a systems or cellular level such that they are more sensitive to stress. AIM 2 will define stress- induced molecular and cellular plasticity that alters LC activity and predisposes to substance abuse. AIM 3 will determine the influence of sex on opiate-induced LC neuronal activity. The proposed work will elucidate how dysregulation of the LC-NE system impacts stress and opiate sensitivity, decisions underlying substance abuse, and sex differences in opiate actions.